The invention relates generally to pen plotters, and more specifically to monitoring and controlling the quality of pen markings on plotting media.
A typical pen plotter comprises a pen for producing markings on a medium in response to instructions from, for example, a computer. The medium such as paper is movable in a first direction along the x-axis and the pen on a main carriage is movable in a second direction along the y-axis which is perpendicular to the the first direction. Thus, by appropriate control of the drives for the pen and paper movement, any desired graphical representation can be produced on the medium. The writing system also typically comprises a rotatable pen carousel carrying a plurality of pens, for example pens of different colors. A pen handling mechanism is provided which permits a pen to move from the pen carousel into a position on the carriage for plotting on the medium and replaces that pen by another one from the carousel, for example when a different color is desired.
Writing systems such as the above-mentioned pen plotter are typically used for producing rather complex graphical representations, for example circuit layouts or construction diagrams, which are electronically stored in a computer on which they may also have been created by a user. Once the plotting of those electronically stored drawings has been initiated by a user, the plotting continues automatically and the user only has to take the completed drawing from the plotter. In practice, however, the completed drawings have not always been satisfactory as certain lines which should have been plotted are missing or have bad quality because the corresponding pen is clogged, or dried out, or is inking out or is in another way not operating properly. As a consequence thereof, the entire drawing generally had to be plotted again, in particular if a pen failure had already occurred at the beginning of the plotting process. Thus, a considerable amount of time is wasted and the efficiency of the plotting is decreased. Usually these problems are resolved by perfecting the components of the plotter so that they are less prone to failures.
In pen plotters, for example, measures are taken to avoid drying out or clogging of the pen. One of those measures is to seal the pen tips by rubber caps while they are in the pen carousel so that they stay moist and ready to write. Despite such attempts for perfection of the writing components, writing failures may still occur, for example, when the ink reservoir of the pen is empty or nearly empty or foreign substances interfere with the writing system or when any other unforeseeable faults occur. Since the plotting typically is performed with high speed, i.e., high relative velocity between the pen and the medium, even small defects in a pen lead to a noticeable degradation of the quality of the drawing.
There have been plotting systems which have a sensor that retraces along the entire actual path of a plotted line and then compares such plotted line with the computer program. Such a system is very inefficient due to the excessive time consumed, and also due to the need for precisely focusing the sensor directly on the plotted line. Also, such a system fails to take into account the different line thicknesses and varied line intensities that result from diverse pen types, colors, plotting speeds and pen force, as well as type of media and ambient lighting conditions.